Conventional implantable leads for use with implantable electrical devices such as cardiac pacemakers and defibrillators are typically constructed of at least one electrical conductor that is preferably wound into a helical form and having an outer insulation layer of tubular form coaxially surrounding the electrical conductor. The tubular insulation is most commonly of an elastomeric material such as silicone or polyurethane. The combination of a helically wound conductor with elastomeric outer insulation provides these conventional constructions with a substantial amount of potential elastic deformation in the direction of the length of the lead.
The fundamental requirements of implantable leads are that they must have excellent mechanical integrity, insulating properties and biocompatibility, and must be flexible with a long flex life to accommodate attachment to a beating heart or other anatomical flexures.
Conventional implantable leads have several disadvantages. The silicone or polyurethane outer coverings are not ideally biocompatible and are frequently known to provoke adverse tissue reactions over time. They are also known to fail due to exposure to blood chemistry. Silicone leads result in cases of acute allergic responses in some patients. Silicone leads promote formation of a fibrous sheath which can ultimately encapsulate the lead. Polyurethane leads frequently fail due to environmental stress cracking and metal ion oxidation. Additionally, these leads are known to break during attempts to remove them from implanted patients by the application of a tensile force. In these cases the remaining portion must be abandoned within the patient's body or must be surgically removed.
Implantable lead wires using insulation materials other than the conventional silicones or polyurethanes have been described previously. U.S. Pat. No. 4,573,480 describes an implantable electrode lead in the form of a helically wound conductor having a tubular insulating layer surrounding the helically wound wire wherein the tubular insulating layer is porous polytetrafluoroethylene (hereinafter PTFE) having a pore size limited to a maximum size described as "being essentially impervious to body fluids to prevent tissue growth thereon." This pore size is described as being not larger than 4 microns. While pore sizes of this range and smaller are known to preclude cellular ingrowth, the material remains pervious to body fluids which will wet out such an insulating layer shortly after implantation. The result is that the effectiveness of the electrical insulation is destroyed. This patent also teaches that the tubular porous PTFE insulating layer may alternatively be provided with an outer covering of smooth and impervious material. While this alternative construction prevents the wetting out of the porous PTFE layer by body fluids, it loses the biocompatible advantage provided by the blood or tissue contacting outer surface of porous PTFE.